This invention relates generally to systems for achieving imaging employing radioisotopes, and more particularly, to a system which obtains and stores an energy distribution in a computer memory as data acquisition progresses, which distribution then is processed to separate same into scattered and direct components.
The purpose of cameras of the type used for radioisotope imaging is to provide an accurate assessment of the distribution of radioisotope within an object from measurements made outside the object. In the particular case of a scintillation camera, gamma rays are detected by a solid crystal and their energy converted into light. The function of the detection circuitry is to determine where in space the gamma ray was detected. A collimation system and a reconstruction algorithm then are employed to obtain a correspondence to be determined between the gamma ray detection location and a location in the object from which the gamma ray originated.
By detecting many gamma rays over time, the distribution of radioisotope emitting the gamma rays is determined. When gamma rays are emitted from deep within an object, however, there is a good possibility that they will undergo one or more Compton scatterings, i.e., changing their direction and losing some energy each time, before they emerge from the object. These gamma rays can therefore approach the scintillation camera from a direction which does not correspond to that of their true origin. If detected and processed as usual, they will be misassigned and the radioisotope distribution which ultimately is obtained will be incorrect.
To reduce effect of this source of error, present cameras determine the energy of the detected gamma rays. If a gamma ray has lost a large amount of energy in scattering, it is not accepted for further processing. Because of inaccuracies in the energy determination, however, a fairly wide window of allowable energies has to be used to accept almost all of the direct (that is unscattered) gamma rays. This wide window leads to the acceptance of a significant number of scattered gamma rays.
The present invention is directed to the rejection of all scattered gamma rays. Thus, only direct gamma rays will be processed and radioisotope distributions will be determined in a way which is qualitatively and quantitatively accurate.
One of the known methods for rejecting scattered gamma rays utilizes the concept of an energy window. This window can be set symmetrically about the peak in the energy distribution (number of gamma rays given as a function of energy of the gamma ray) or it can be set asymmetrically about that peak. In addition, one scheme has proposed obtaining energy distributions for different spatial locations on the planar face of the scintillation camera and using different energy windows for different spatial locations. This scheme is disclosed in U.S. Pat. No. 4,212,016 which issued Jul. 8, 1980 to Knoll, Strange, and Bennett and entitled "Radiation Signal Processing System."
A further known method for rejection of such scattered gamma rays utilizes two energy windows: one set symmetrically about the direct peak and one of equal width set adjacent to that window but at lower energies. A fraction of the image of the radioisotope distribution that is reconstructed from gamma rays accepted within this reduced-energy window is subtracted from the image reconstructed using the direct, Le., unscattered, gamma rays.
A related method for handling the problem of Compton-scattered gamma rays involves calculating their true origin. This reconstruction method is called "Inverse Monte Carlo" reconstruction. It solves the photon transport equation for single photon emission computed tomography acquisitions.
The method disclosed in U.S. Pat. No. 4,839,808, which issued Jun. 13, 1989 in the names of some of the inventors herein, involved acquiring a full energy spectrum at every spatial location on the camera. In a calibration step, the spectrum from a pure radioisotope source, which was small in extent so as to prevent scattering, was also utilized. A minimization was then carried out for the difference of two functions to obtain a function for the unscattered gamma rays. In the known grandparent invention, the second function in the difference depends linearly on its parameters and so the method of linear least squares was employed in the minimization.
In a continuation-in-part parent patent application filed Mar. 31, 1989, (U.S. Ser. No. 07/331,993 now U.S. Pat. No. 5,081,581 the second function in the-difference depends non-linearly on its parameters and therefore non-linear least squares was utilized.
U.S. Pat. No. 4,575,810 which issued Mar. 11, 1986 to Everett W. Stoub and entitled "Method and Circuit for Processing Pulses by Applying the Technique of Weighted Acquisition", proposed detecting the energy content of gamma rays from an internal radioactive decay after emergence from the body for the purpose of correcting for gamma-ray scattering. This known technique appears to be directed only to achieving improvement in image contrast, and permits bias in their estimates.
In a further known system for detecting the energy content of gamma rays for Compton scattering correction, the processing method is based on multivariate-analysis techniques. In yet another known system, processing is based on constrained factor analysis.
It is a problem with the known systems and methods that the use of a single energy window does not lead to the complete rejection of gamma rays which have undergone Compton scattering. More specifically, as the window is made more narrow or is set asymmetrically toward higher energies, the ratio of scattered gamma rays over direct gamma rays is made smaller but the total number of direct gamma rays which are accepted is made smaller also. Statistical fluctuations in the determined radioisotope distribution get worse as the number of direct gamma rays gets smaller. Therefore, to avoid unacceptable statistical fluctuations, a significant number of scattered gamma rays usually have to be accepted. The use of energy windows which vary with spatial location improves the situation, but does not solve the underlying problem.
In the known systems which use two adjacent energy windows, one investigator has noted that the fraction, k, used to multiply the image from the lower window before subtraction from the upper window image is dependent on the source distribution. In support of this contention, it is noted that employing on-axis and off-axis (8 cm from the axis of the cylinder) line sources placed within 22 cm diameter cylindrical phantom, yields values of k which have been experimentally measured to be equal to 0.66 and 0.47 for the on-axis and off-axis positions, respectively. This variation of the k fraction with the radioisotope distribution means that it is impossible to obtain the radioisotope distribution in a straight-forward or practical manner. In other words, one can find the correct k value only if one knows the distribution, and one can only find the distribution if one already knows the k value.
A characteristic of the Inverse Monte Carlo method appears to be that the cross section for absorption and the cross section for scattering must be known independently for all points within the object. In addition, the Inverse Monte Carlo method is quite slow in producing an image with present-day computing facilities. This slowness appears to be inherent in the method since the total object must be reconstructed for the method to work accurately.
The method disclosed in the aforementioned U.S. Pat. No. 4,575,810 to Stoub requires measuring camera energy-spatial spread functions for point sources imbedded in a particular scattering medium. This scattering medium can only be an approximation to the true situation in patients and, accordingly, there must be bias errors in the estimates therein disclosed.
The aforementioned linear least squares method is sensitive to statistical noise in the measurement of the spectra. Grouping of the data has been applied to reduce this noise. In a practical image, such grouping can lead to a difficult interpolation problem to get back to the fine grid needed for reconstruction. Simulations have shown that the linear least squares estimates are also sensitive to the choice made for the range of the energy spectra used in fitting (called the "fitting window"). In practical situations, the optimum choice is not known, and a suboptimal choice may be made leading to error in the estimate.
Lastly, the linear least squares and the non-linear least squares systems both require post-processing solutions to the minimization. This post-processing requirement means that the solution can be available only some finite time after data acquisition. The patient dismissal from the imaging area is delayed by this amount of time, resulting in additional expense and diminution of resource utilization.
It is, therefore, an object of this invention to provide a radioisotope imaging system which rejects all scattered gamma rays.
It is another object of this invention to provide an imaging system that eliminates all scatter and thereby provides an unbiased estimate.
It is also an object of this invention to provide a system wherein rejection of scattered gamma rays can be accomplished for any distribution of radioisotope in the object or patient.
It is a further object of this invention to provide a system in which computation time is feasible.
It is an additional object of this invention to provide a system which is operable in real time.
It is another object of this invention to provide a system which allows for energy shifts between the spectrum of direct gamma rays corresponding to the scatter-free source calibration, and the local spectra of direct gamma rays acquired from the patient or object.
It is additionally an object of this invention to provide a system which is applicable to single-photon projection (planar) imaging.
It is yet a further object of this invention to provide a system which is applicable to single-photon emission computed tomography.
It is also another object of this invention to provide a system which is applicable to positron emission computed tomography.
It is yet an additional object of this invention to provide a system which is applicable to NaI (T1), BgO and other scintillation materials.
It is still another object of this invention to provide a system wherein quantitatively accurate image reconstruction is achieved.
It is a yet further object of this invention to provide a system in which all of the direct (unscattered) gamma rays which are detected in the crystal are used for formation of the image.
It is also a further object of this invention to provide a system which can be applied to any number of tomographically-reconstructed planes through the body.
It is additionally another object of this invention to provide a system which reduces computation time when only a relatively small number of planes are of interest.
A still further object of this invention is to provide a system which is insensitive to noise.
An additional object of this invention is to provide a system which remains stable when noise is present in the data.